The present invention relates to a gas container, a coupling for the gas container, and a method of making the container. The coupling is arranged between the gas container and a valve comprising a nut and cooperating flange surfaces. The container is arranged to contain pure gases under high pressure and comprises a top part, a bottom part and a mantle. The top part is provided with an externally threaded neck. The method relates to the manufacturing of a metallic container intended for high pressure applications, preferably a container of stainless steel for enclosing of pure gases.
In certain gas applications in medicine, electronics and the pharmaceutical industry there are extremely high demands upon the purity of the gases. Due to the purity demands, the surfaces of articles or material coming into contact with the gases have to be prepared in such a way that the risk of contamination, for example by loose particles on the inner surfaces of the equipment, is eliminated.
One traditional technique of manufacturing gas containers involves so-called "spinning" of the ends of seamless tubes at high temperature, whereby, the entire container is prepared as one integral piece. Usually, high-strength low-alloyed Cr-Mo steels are used which require heat treatment after the deformation process in order to produce the desired strength properties. The design pressure of this kind of containers can be several hundred bar.
The problem with these known containers is that it has been very difficult to make the extremely smooth surfaces which the applications of pure gas demand Furthermore, the carbon steels in themselves present a potential impurity risk because that material not corrosion resistant. Therefore, certain gases can react with the steel surface if moisture is present and will form corrosion products which are unacceptable from the point of view of purity.
Until now, it has been found impossible to solve the above-mentioned problems by using conventional techniques, because of the risks of "pipe" appearing in the bottom during the welding and because of a rough surface which is formed in the zone where the container becomes narrower towards the valve opening in the top of the container.
Conventional valve connections for gas containers are usually provided with conical threads made tight with some kind of packing material, TEFLON.RTM. or the like. This kind of thread demands great care during the connection of the valve in order to obtain proper sealing, and the direct contact between gas, packing and thread root means a potential impurity source both during the evacuation of the container at increased temperatures and during its use.
A coupling whose purpose is essentially different from the aim of the present invention and which is designed to be welded on a container for liquids is described in U.S. Pat. No. 4,316,624. That coupling comprises a connection part, an adapter and a nut. The adapter supports a tube being threaded in one part wherein an unavoidable clearance is formed between the ends of the adapter and the tube which can be occupied by impurities. The connection part is welded to a tube or similar in which liquid flows. Couplings are generally subjected to bending stresses, and the location of the weld in the direct connection of the coupling is therefore unfavorable. The weld is also difficult to work because of poor accessibility.
An object of the present invention is to provide a coupling for a gas container which resists a contamination of pure gases.
Another object of the present invention is to provide a coupling for a gas container which is insensitive to the carefulness of the mounting.
A further object of the present invention is to provide a gas container which resists contamination of pure gases.
Another purpose of the present invention is to provide a method of making a gas container for enclosing pure gases and intended for high pressure applications.